The evolution of sedimentary systems during the Mesozoic and Cenozoic in southern Alaska can be divided into six episodes, each representing a tectonic cycle that is presently recorded, with varying degrees of clarity, as a plutonic belt containing related sedimentary rocks that are bounded by unconformities or faults. These six episodes or tectonic cycles, which are sequential but sometimes overlap, occurred during the: (1) Permian(?) through Early Jurassic, (2) Early Jurassic into Early Cretaceous, (3) late Early and early Late(?) Cretaceous, (4) Late Cretaceous, (5) early Cenozoic, and (6) late Cenozoic.

Each tectonic cycle began with magmatism and volcanism that constructed a magmatic arc with a back-arc and fore-arc basin, outer-arc ridge, shelf, slope, trench, and abyssal plain. Uplift and erosion dissected the magmatic arc and outer-arc ridge, and sediment filled the arc basins, shelf, slope, and trench, and then spilled onto the abyssal plain. Throughout the cycle, tectonism, magmatism, and sedimentation were ongoing at varying intensities, each cycle or episode affected the previous cycle. This evolution can be traced in the composition of the plutonic bodies, conglomerates, and sandstones. Thus, southern Alaska has evolved from young continental and oceanic crust to mature continental crust by magmatism, sedimentation, and accretion.

Paleomagnetic data suggest that much of southern Alaska south of the Denali fault evolved as one or more terranes or plates in southerly latitudes from Permian(?) time that were accreted to North American continental terranes by early Cenozoic time. The temporal and spatial relation of these terranes throughout geologic time in relation to the tectonic cycles is intricate and complicated. The Alaska-Aleutian Range volcanoplutonic arc and the associated Queen Charlotte transition zone transform-fault system have dominated depositional and structural patterns in southern Alaska since the early Cenozoic.

End_of_Article - Last_Page 953------------